There are previously known apparatus and methods for detecting intrusion into a space. Common detectors include door and window contacts, glass break detectors, and motion sensors. Motion detectors typically employ ultrasound, passive infrared ("PIR"), or radar detection techniques. Ultrasonic motion detectors are commonly used for automatic door openers and security alarms. They are inexpensive and can operate with narrow beamwidths. However, installation options are limited because ultrasonic beams are easily blocked by thin materials, including paper. Another disadvantage is false triggering caused by reflections from blowing curtains, pets, and flying insects.
PIR motion detectors are perhaps the most frequently used home security device. They employ a special Fresnel lens to generate multiple thermal images of a warm object, such as a person. As the person traverses the PIR field of view, the thermal images produce periodic fluctuations that are detectable by inexpensive electronics. As with ultrasound detectors, PIR detectors are disadvantageous because they can be blocked by a sheet of paper. Moreover, PIR detectors have no range adjustment and may be false-triggered by pets or rodents.
Radar-based motion detectors may emit a continuous-wave ("CW") microwave signal and compare the emitted and echo frequencies to produce a beat frequency that is proportional to range. Such detectors employ microwave oscillators, detector diodes, and audio frequency processing electronics to detect objects. Nonranging radar detectors emit a CW microwave carrier and detect a Doppler shifted echo from a sensed moving object. Such swept and Doppler radar-based motion detectors are disadvantageous because of limited materials penetration, microphonics, frequency crowding, poor short-range operation, and difficulty meeting Federal Communications Commission ("FCC") testing and certification requirements.
Some of the disadvantages of swept and Doppler radar-based motion detectors have been circumvented by another radar-based motion sensing technique. U.S. Pat. No. 5,361,070 for ULTRA-WIDEBAND RADAR MOTION SENSOR describes ultra-wideband ("UWB") radar motion sensing in which a UWB radar operates as a pulse-echo system that clocks the two-way time of flight of a very short electrical pulse. Rather than employing a carrier frequency, a fast electrical voltage impulse is applied directly to an antenna. The resulting spread spectrum emissions resemble the Fourier transform of the emitted pulse and generally span hundreds of megaHertz to several gigaHertz. Because most materials exhibit rapidly increasing attenuation with frequency, UWB radar is advantageous for materials penetration, allowing installation of UWB radar detectors behind walls and appliance panels, above ceilings, and below floors. UWB radar motion detectors are also advantageous for detecting close objects and may have an adjustable sensing range. The patent asserts that "a frequency allocation by the FCC is not relevant" because of the UWB emissions. However, UWB radar-based motion detectors have not attained market acceptance because they have not readily met FCC regulatory limits as an intentional radiator, a fact that is well documented in "The Development and Commercialization of Micropower Impulse Radar at Lawrence Livermore National Laboratory", a report by the Democratic Staff Committee on Science, U.S. House of Representatives, Apr. 12, 1999.